MdePkg/Test: Add SafeIntLib and BaseLib Base64 unit tests

https://bugzilla.tianocore.org/show_bug.cgi?id=2505



* Add unit tests for SafeIntLib class
* Add unit tests for BaseLib Base64 conversion APIs.
* Add Test/MdePkgHostTest.dsc -to build host based unit
  tests
* Update MdePkg.dsc to build target based tests for
  SafeIntLib and BaseLib
* Update MdePkg.ci.yaml to build and run host based
  tests for SafeIntLib and BaseLib

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Bret Barkelew <Bret.Barkelew@microsoft.com>